Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $63.53   3.311%   $65.6335   $65.63   $65.65   $65.60  
  Purchase #2 $76.53   8.827%   $83.2853   $83.29   $83.30   $83.30  
  Purchase #3 $245.68   10.180%   $270.6902   $270.69   $270.70   $270.70  
  Purchase #4 $68.51   10.337%   $75.5919   $75.59   $75.60   $75.60  
  Purchase #5 $133.25   3.433%   $137.8245   $137.82   $137.80   $137.80  
  Purchase #6 $89.26   3.374%   $92.2716   $92.27   $92.25   $92.30  
  Purchase #7 $66.57   12.151%   $74.6589   $74.66   $74.65   $74.70  
  7 purchase totals = $743.33       $799.9559   $799.95   $799.95   $800.00